1. Field of the invention
The invention relates to an optical beam scanner used for a recording apparatus such as a laser beam recording apparatus, in which a rotating polygon mirror is fixed to a rotating shaft that rotates at a high speed.
2. Related art
As shown in FIG. 4, an example of such a conventional optical beam scanner 1 includes a prismatic rotating polygon mirror 2 with lateral surfaces being formed into mirrors and a brushless motor 4 that rotates a rotating shaft 3, to which the rotating polygon mirror 2 is fixed, at a high speed (see e.g., Japanese Utility Model Examined Publication No. 63-1287).
The brushless motor 4 has: cylindrical casings 5a, 5b (the casing 5b is the bottom), which are put one upon another in two stages; a bearing housing 6 fixed to the upper casing 5a and supporting the rotating shaft 3 through bearings 6a, 6a; a stator 7 fixed to the outer circumference of the bearing housing 6 and having a coil 7a wound around salient poles; a magnet 8a fixed to the inner circumferential surface of a yoke 8 so as to confront the stator 7; a disk 9 attached to the bottom end of the rotating shaft 3; and a rotation detecting sensor 10 confronting the disk 9. When the coil 7a is energized, the magnet 8a rotates integrally with the rotating shaft 3, which in turn rotates the rotating polygon mirror 3 at a high speed to start scanning with an optical beam.
The rotating drive used for an optical beam scanner must satisfy a stringent requirement that the rotating drive not only is free from inconsistent rotation and deviation of rotation but also can rotate at a high speed (4000 rpm or more) with no splash of oil from the bearing section.
However, since the thus constructed optical beam scanner 1 is designed to rotate the yoke 8 integrally with the rotating shaft 3 in order to meet the above requirement, the rotating shaft 3 does not balance well. In addition, a drive circuit (not shown) for controlling conduction to the coil 7a based on the result of the rotation detection from the sensor 10 must be provided, which impairs the performance of the scanner with elevated parts cost and increased current loss within the circuit due to heating of the drive circuit. Moreover, the gap between the yoke 8 and the upper casing 5a encourages noise leakage and makes handling difficult.